Central nervous pathways and control of the airways.

Neural control of airway muscles and secretions is predominantly by excitatory parasympathetic and non-adrenergic, non-cholinergic innervations (excitatory and/or inhibitory depending on the species). Functionally distinct afferents effecting airway reflexes terminate in different but overlapping parts of the nucleus tractus solitarius, where integration of simultaneously evoked reflex responses occurs. Parasympathetic preganglionic neurones are located in the dorsal vagal nucleus and nucleus ambiguus, which also contains upper airway motoneurones. These output neurones receive inputs from the central respiratory network which modify the effectiveness of reflex activity. This is particularly important since many afferents evoking airway reflexes concurrently modify respiratory drive. Thus, their effect on the outflow is twofold, a direct reflex effect and an indirect respiratory action and these may facilitate or antagonise one another. Although there is reflex control of individual motor outflows, in some defined situations, e.g. swallowing and coughing a stereotypical pattern of motor outflow is evoked. The neural mechanisms underlying these aspects of airway control are discussed.